1. Field of the Invention
This invention relates to optical resonators and, more particularly, to such resonators that are designed to include chaotic optical ray paths (either fully chaotic or quasi-chaotic ray paths) and, in addition, to the use of such resonators in conjunction with lasers for trace-gas (e.g., pollution) monitoring or optical amplification.
2. Discussion of the Related Art
Optical cavity resonators are devices having internal optical path lengths that are much longer than their physical dimensions. The long optical path lengths are produced by multiple reflections of optical rays at mirror surfaces. Cavity resonators find widespread application in diverse fields; for example, in amplifiers for high power laser systems and in ultra-high sensitivity gas sensing systems.
In the case of atmospheric pollution sensors, for example, a sample of polluted air is introduced into a chamber, and a light beam of suitable wavelength is passed through the sample. The wavelength is chosen to correspond to an absorption band/line of the particular pollutant that is suspected to be in the atmosphere. However, because such pollutants are often present in trace amounts, the amount of absorption in a single pass of the beam is extremely small and, in fact, is often not above the noise level of the detection system. To be detected with a sufficient signal-to-noise ratio the amplitude modulation of the beam produced by the trace-gas absorption must be greater than the background noise, which is nearly impossible to achieve in a single pass. Consequently, the prior art has resorted to the use of an optical cavity resonator to contain the sample so that absorption by the trace pollutant can take place on each of a multiplicity of passes of the beam through the sample.
Typically, a total optical path length (TOPL) from about several meters to a few hundred meters or a few kilometers is desired for the detection of many trace gases (e.g., CO, NOx, NH3, CH4, their isotopes, water vapor isotopes, and others) in the atmosphere at signal-to-noise ratios of at least 2 or at a sensitivity level of few parts per billion. However, to achieve such TOPLs the prior art has resorted to resonators that have very large physical dimensions; e.g., the resonator described by D. Horn et al. [Appl. Opt. Vol. 10, No. 8, pp. 1892-1898, (1971)] utilized 254 reflections along a 10 m base path for a TOPL of 2.54 km. Such designs pose several problems. First, the number of reflections is limited by the presence of various ports (e.g., gas ports, optical ports) that interrupt the reflective surfaces. Second, the TOPL of 2.54 km was achieved only with an unwieldy, 10-m-long apparatus. Third, long apparatus of this type typically has low gas throughput.
Consequently, there is a need in the trace-gas sensing art for a more compact optical cavity resonator that can achieve the relatively long TOPLS necessary for trace gas monitoring. Advantageously, the compactness of the resonator would also provide for a relatively rapid gas cycle time.
Resonators with long TOPLs also find application in optical amplifiers for high power laser systems. A signal laser beam to be amplified is injected into a resonator that contains a gain medium. The beam undergoes multiple reflections within the resonator. Therefore, the longer the TOPL traversed by the beam, the higher the total gain it experiences. However, if the injected beam traverses a closed path, it will eventually deplete the gain medium, reducing its ability to provide gain. This phenomenon is sometimes referred to as gain depletion.
Thus, a need remains in the optical amplifier art for a resonator that is compact, yet has a relatively long TOPL, and that exhibits open paths so that the gain medium does not exhibit gain depletion to any significant extent.